Starret Dial Indicator Thread

A while back I picked up a Starret dial indicator. Instead of a mounting tab with a hole through it like my cheap indicator it has a threaded socket
on the back. It looks like about 3/8, but its not 3/8 16. Anybody know off hand what the thread is?
I know I should just grab a thread gage and check, and I will when I am out in the shop later, but right now I am in the office, and I am headed out to do a service call.
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It kinda looks like 3/8 24, but a cheap tap felt awfully stick after only about three threads engaged. I didn't want to push it and damage the threads. I don't have a 3/8 24 bolt laying around to check it with. I'm not happy with the "feel" of a thread gage in it. None felt quite right.
It says 25-441J on the box, but it only says 25-441 on the dial. They do not show a 25-441(no J) as a model.
Interestingly when I looked up the 25-441J on Starrett's website it has a conventional lug style back. Not the threaded socket like the one I have.
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wrote:

Having been on the phone with two people from Starrett today, and having met with another one in person after lunch, I can testify that they do have good technical people there who will answer the phone. d8-)
Why don't you give them a call and get it straight from the horse's mouth?
--
Ed Huntress

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I did get an email back from Starrett first thing this morning. I had already made my mounting stud late last night. I need to just throw that box of cheap taps away. LOL. I pulled out an older box of taps with thread gages that I have, and the tap spun right in. The thread gage felt better too. It is a 3/8-24. I've used the dial indicator, and put it away already.
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wrote:

Aha, good news.
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Ed Huntress

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wrote:

The backs are changeable and can be ordered separately from the indicator. The "Screw Type Lug" back is available with 1/4-28 and 3/8-24 threads from Starrett. All bets are off if it's not a Starrett back. The backs are swappable among some brands, others are not.
J appears to indicate a plain no-lug back.
--
Ned Simmons

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    Hmm ... M9.5x1 thread instead? I don't think that is a standard thread, but there are lots of alternative threads made for special purposes.

    The 'J' may be a suffix indicating a non-standard backplate. There are quite a few options available to fit various setups.

    So -- the box and indicator have had an alternative back added after sale? IIRC, there are four small screws around the edge securing the back plate (and allowing the lug style to be rotated so the lug is vertical or horizontal.)
    So -- just get a replacement back plate from Starrett if that would make you happy.
    Enjoy,         DoN.
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LOL. But I would still be waiting to true up my new spindle mount on the Taig instead of sitting in my office watching it on on the camera while it cuts.
Turns it it was a 3/8 24. The tap I tried it with the first time was form a cheap set somebody gave me. When I checked it with a decent tap it spun right in. I cut my mounting stud last night late, and trued up my mount this morning.
I gotta throw away that cheap set of taps. LOL.
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    [ ... ]

    Understood.
    Or figure someone you dislike enough to give the set to him. :-)
    What brand was that set, so we can avoid it? :-)
    Enjoy,         DoN.
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